This invention relates to air/fuel induction systems for spark ignition internal combustion engines. Such systems comprise an air/fuel induction passage with a driver-operable throttle valve therein, and a fuel system by which gaseous fuel is presented to a location in the induction passage, the gaseous fuel being vaporized liquefied petroleum gas (LPG), or a natural gas, such as methane. This invention also relates to electromagnetic valves which are suitable for use in such systems.
Equipment for fueling motor vehicle spark ignition internal combustion engines with LPG which is available commercially at present is in the form of conversion kits by which an existing motor vehicle spark ignition internal combustion engine installation which includes a petrol supply system incorporating a carburetter is converted to either an LPG supply system or a dual fuel system such that the engine can be fueled by either LPG or petrol at the choice of the driver.
Such commercially available LPG fuel system equipment comprises an LPG storage tank, a vaporizer, a vaporized LPG pressure regulator, and a mixer unit. LPG is stored under pressure in liquid form in the tank. The vaporizer is connected between the interior of the tank and the mixer unit which in turn is fitted to the carburetter so that it communicates with the carburetter induction passage when the equipment is fitted. A shut off valve is provided between the vaporizer and the LPG tank when the engine installation is converted into a dual fuel system and that shut off valve is normally closed but is opened automatically by the action of the driver selecting LPG fuel for fueling the engine. The vaporizer is adapted to vaporize liquefied petroleum gas fed to it from the storage tank, the vaporization being effected by reducing the pressure of the liquefied petroleum gas and bringing it into a heat exchange relationship with water tapped from the engine cooling system, the vaporized LPG being collected in a chamber which is in direct communication with the carburetter induction passage via the mixer unit. The pressure regulator includes means whereby the pressure of vapour in that chamber is maintained at a substantially constant near atmospheric pressure. Hence vaporized LPG is drawn into the carburetter induction passage via the mixer unit by engine suction and is mixed with the air flow through the induction passage to form the air/fuel mixture that is fed to the engine by the carburetter in the usual way. The amount of vaporized LPG drawn into the carburetter induction passage is controlled by operation of the mixer unit which is controlled by the demand signal that comprises engine suction. German Offenlegungsschrift No. 2131804 and U.S. Pat. Nos. 3,960,126 and 4,020,810 are concerned with such LPG fuel systems.
Such commercially available LPG fuel system conversion equipment rarely leads to the true potential of LPG as a low emission fuel being realised. The functional performance characteristics of this equipment are inadequately matched to the requirements of the engine. Even though the engine is tolerant, there are problems on fuel enrichment. Also the low temperature operation of the equipment is often unsatisfactory.
The equipment is critically dependent upon the mixer units but it has proved difficult to design mixer units which are sufficiently flexible in their application for them to be properly matched with the engine's requirements. Furthermore the mixer units of the LPG conversion equipment can significantly influence the operation of the basic petrol fuel system in an undesirable manner.
The fact that the delivery stage of the part of the equipment that includes the vaporizer and pressure regulator operates at a low, near atmospheric pressure leads to a requirement for large "active" areas but the resultant design compromise limits accuracy, sensitivity, dynamic response and durability of the equipment. Attempts to minimise these functional deficiencies have tended to increase the level of complexity of the mechanically operable pressure regulating equipment. German Offenlegungsschrift No. 2131804 and U.S. Pat. No. 3,960,126 disclose systems which incorporate complicated two stage pressure regulators which operate to vary the pressure of the vaporized fuel within a narrow range of pressures which are substantially constant and near atmospheric pressure and which comprise a control pressure chamber at ambient pressure which is bounded by two diaphragms and which separates the vaporized fuel from another chamber which is in communication with the engine air/fuel induction passage downstream of the driver-operable throttle valve. U.S. Pat. No. 4,020,810 discloses the use of an economiser valve which is responsive to pressure regions in the engine air/fuel induction passage upstream from and downstream from the driver-operable throttle valve, and which is effective to modify the pressure in a control pressure chamber of a vaporized fuel pressure regulator to cause it, under certain conditions, to lean out or enrich the charge supplied from the pressure regulator to a conventional mixing valve which supplies air according to engine demand.
An object of this invention is to provide fuel system equipment for fueling a motor vehicle spark ignition internal combustion engine with gaseous fuel, such as LPG, the equipment having functional performance characteristics which are more adequately matched to engine requirements than are those of commercial LPG fuel system equipment currently available, and being relatively simple from the mechanical viewpoint.
According to one aspect of this invention there is provided an air/fuel induction system for a spark ignition internal combustion engine, the system comprising an air/fuel induction passage with a driver-operable throttle valve therein, and a fuel system by which gaseous fuel is presented to a location in the induction passage; the fuel system comprising pressure regulating means operable to regulate the pressure of the gaseous fuel and conduit means by which the gaseous fuel is conveyed to said location, the pressure regulating means being adapted to respond to changes in a depression which is established downstream of the throttle valve by operation of the engine and to effect a consequent change in the regulated pressure of the gaseous fuel such that it varies in the opposite sense to variations of said depression; wherein the pressure regulating means are adapted to maintain the gaseous fuel at a pressure higher than that established at said location by operation of the engine and comprise a single movable wall which separates two chambers, one of the two chambers containing gaseous fuel at the regulated pressure and the other chamber being in communication with said induction passage downstream of said throttle valve, there being an injection valve in said conduit means operable to effect injection of metered quantities of said gaseous fuel whereby said gaseous fuel is presented to said location, and control means responsive to certain engine operating conditions and operable to control operation of said injection valve in accordance with those conditions such that the volumetric flow rate at which said gaseous fuel is injected for presentation to said location is matched to the operational requirements of the engine.
Accordingly, in operation of the present invention, instead of presenting fuel vapour to the carburetter induction passage via a mixer unit and relying on engine suction to draw that vapour into the induction passage via the mixer unit from the delivery chamber wherein the pressure of the vapour is maintained substantially constant and near atmospheric pressure by operation of the pressure regulator, we maintain and regulate the pressure of gaseous fuel in the delivery stage at a higher level which we vary automatically in accordance with, but in the opposite sense to, changes in the engine inlet manifold depression by means of a simple single stage pressure regulator, and we inject that gaseous fuel into the carburetter induction passage, whilst controlling the opening of the injection valve automatically in accordance with certain measured engine operating parameters, such as engine speed and engine load. By varying the gaseous fuel injection pressure in accordance with engine operating conditions as well as controlling the opening of the injection valve in accordance with measured engine operating conditions, we are better able to match the performance of the fuel system to the requirements of the engine than is possible by merely controlling the valve opening. The gaseous fuel fed to the injection valve should be a dry gaseous fuel without any liquid phase otherwise uniform fuel metering cannot reasonably be expected. The range of control of such an injection valve is limited in practice to such an extent that it is insufficient to adequately match the supply of a gaseous fuel to the requirements of the engine over the full range of engine operation if that gaseous fuel is injected at a constant pressure.
Preferably the pressure regulating means include a pressure relief valve operable to vent said one chamber into said other chamber and hence to said induction passage downstream of said throttle valve, should the pressure in said one chamber exceed a predetermined maximum.
Conveniently the injector valve is controlled electromagnetically by said control means. It may be a digital valve, that is to say a valve which is so controlled for digital operation, viz. continual valve opening and closing at a controlled frequency and duration of operation. Such a digital valve is advantageous as compared with a simple analogue valve since friction is not a significant influence on its operation. However the output from a digital injection valve is a pulsed flow of gaseous fuel and we have not been able to synchronise that pulsed flow with operation of the engine so that fuel distribution problems follow from the use of a digital injection valve. With these difficulties in mind we prefer to make provision for smoothing the pulsed output of the digital injection valve prior to its presentation to said location in said induction passage. Such provision may include arranging for said conduit means to communicate with said induction passage through a diffuser arrangement which circumferentially surrounds said induction passage at said location and which causes the pulsed output of the digital injection valve to circulate around the induction passage and be spread circumferentially therearound for presentation to said location through a circumferentially-extending inlet provided. Additionally or alternatively the dimensions of the flow path between the digital injection valve and the induction passage may be selected so that smoothing of the pulsed output from the digital injector valve is effected in that flowpath.
The control means may comprise an analogue valve drive circuit adapted to process an electrical input signal which is the product of electrical signals indicative of measured values of engine speed and engine load, into a valve driving signal, or a digital drive circuit including a microprocessor having a data store matrix which is addressed by the electrical signals indicative of measured values of engine speed and engine load, the microprocessor responding automatically to provide the appropriate valve driving signal derived from data stored in the matrix.
If the injection valve is a digital valve and it is operated at a constant frequency, the opening pulse width being selected in accordance with control signals derived from signals indicative of measured values of engine speed and engine load, its dynamic range is not sufficiently wide for it to respond to all the control signals that are applied to it. With this problem in mind, we prefer that the digital valve control means are arranged so that, for all operations of the digital valve below a predetermined fuel demand requirement of the engine, the valve is operated with a constant opening pulse width at a respective frequency selected automatically from within the range of operating frequencies of the valve in accordance with the measured values of engine speed and engine load, whereas for all operations of the valve at or above said predetermined fuel demand requirement, the valve is operated at maximum frequency, the opening pulse width being selected automatically in accordance with the measured values of engine speed and engine load. Controlling the digital injection valve in this way results in its having a wider dynamic range than it would have if it was always operated at maximum frequency.
U.S. Pat. No. 4,141,326 discloses a closed loop fuel control system for a hydrogen fuelled engine which electronically controls fuel delivery to the engine in response to signals indicative of the engine's operating parameters and a signal generated by a hydrogen sensor in the exhaust manifold to maintain the concentration of hydrogen in the exhaust at a predetermined level. The means of metering the flow of hydrogen to the engine comprises an electrically controlled valve of digital or analogue type. Various analogue circuits for driving the valve are described as is the use of digital electronics.
According to another aspect of this invention there is provided an air/fuel induction system for a spark ignition internal combustion engine, the system comprising an air/fuel induction passage with a driver-operable throttle valve therein, and a fuel system by which gaseous fuel is presented to a location in the induction passage; the fuel system including conduit means by which the gaseous fuel is conveyed to said location, a digital injection valve in said conduit means operable to effect injection of metered quantities of said gaseous fuel whereby said gaseous fuel is presented to said location, and control means responsive to certain engine operating conditions and operable to control operation of said digital injection valve in accordance with those conditions such that the volumetric flow rate at which said gaseous fuel is injected for presentation to said location is matched to the operational requirements of the engine; wherein gaseous fuel flow smoothing means are provided within said conduit means between said injection valve and said induction passage whereby the pulsed output of the digital injection valve is smoothed prior to its presentation to said location in said induction passage.
According to another aspect of this invention an electromagnetic digital valve is controlled so that, for all operations below a predetermined "mark space ratio" (M.S.R.), it is operated with a constant opening pulse width at a respective frequency selected automatically from within the range of operating frequencies of the valve in accordance with certain external control parameters, whereas, for all operations at or above said predetermined M.S.R., it is operated at a maximum frequency, the opening pulse width being selected automatically in accordance with the external control parameters.
According to yet another aspect of this invention, operation of an internal combustion engine digital electromagnetic fluid fuel injection valve is controlled at a frequency which is not synchronised with the speed of the engine by a control arrangement comprising a microprocessor having a data store matrix which is addressed by electrical signals indicative of measured values of engine speed and engine load, the microprocessor responding automatically to determine the frequency and pulse width of the digital valve driving signal from data stored in the matrix, the valve having a range of possible operating frequencies and pulse widths.